Electron discharge tube



Oct. 2, 1951 J. w. SKEHAN ETAL 9,87

ELECTRON DISCHARGE TUBE Filed Dec. 24, 1949 3 Sheets-Sheet 1 ATTORNEY-51951 J. w. SKEHAN ETAL 2,569,872

ELECTRON DISCHARGE TUBE Filed Dec. 24, 1949 3 Sheets-Sheet 2 INVENT R 9ATTORNEYS 51 J. w. SKEHAN m 2,569 872 ELECTRON DISCHARGE TUBE Filed Dec.24, 1949 3 Sheets-Sheet 3 INVENT f 1 -4 w. 3216 sy iutl' S Bau (CAM;31416; W

ATTORNEYS Patented Oct. 2, 1951 ELECTRON DISCHARGE TUBE Joseph W.Skehan, Stamford, and Thomas H.

Rogers, New

Canaan, Machlett Laboratories, Incorporated, Spring- Conn., assignors todale, Conn., a corporation of Connecticut Application December 24, 1949,Serial No. 134,926

19 Claims. (01.313-57) This invention relates to electron dischargedevices of the type disclosed in Horsley Patent No. 2,482,275, issuedSeptember 20, 1949, in which electrons emitted by an annular filamentare focused to cause them to pass through the area defined by thefilament and to form a beam impinging upon'a target surface, andradiation produced by the target passes through the area within thefilament and through a window in the tube envelope. More particularly,the invention is concerned with a novel electron discharge deviceinvolving the use of the Horsley invention, in which the electron beamis caused to scan the target surface and means are provided forconverting into a useful beam the radiation produced at the target byimpingement of the electron beam. The new discharge device may beemployed advantageously for X-ray production or as a cathode ray tube inprojection television.

A form of the new device producing X-rays may be used for radiologicalpurposes and, with especial benefit, in fluoroscopic examinations. Insuch examinations as now carried on, the X- ray tube employed producesX-rays in the form of a cone of considerable size and the rays-passsimultaneously through all parts of the object under examination andfall upon a fluoroscopic screen. The fluoroscopic image appearing on thescreen then results from differential absorption of the X-rays by thedifferent parts of the object. With all parts of the object thus.simultaneously irradiated, the image on the screen lacks brightness anddefinition to such an extent that fluoroscopy is not satisfactory formany purposes. The lack of brightness cannot be overcome by merelyincreasing the intensity of the X-ray beam, because the patient'stolerance to X-ray exposure would too quickly be exceeded. The proposalhas been made, in order to overcome these objections, that the X-raybeam employed in fiuoroscopy be of small size and be caused to scan theobject, so that the different parts of the object will be traversedconsecutively by the rays, and the X-radiation, after transmissionthrough the object, will fall upon a block of fluorescent material,causing it give off visible and ultra-violet light modulated inintensity in accordance with the differences in absorption of the X-raybeam by difierent parts of the object.

The light would then be focused and allowed to fall upon thelight-sensitive element of a photomultiplier tube, and the resultantsignal would be amplified by a suitable vacuum tube amplifier and fedinto a television receiving system, whose sweep-circuit would besynchronized with the scanning sequence of the X-ray'source, so that theX-ray image would be presented on a cath-. ode-ray tube screen and itsbrightness could be controlled independently of the intensity of theX-ray beam.

It appears that the various components necessary for the successfulperformance of the proposed system are available in reasonablysatisfactory form, with the exception of an X-ray tube capable ofproducing a scanning beamof X-rays of suitably small cross-section area,adequate intensity, and suitable pattern of scanning. The presentinvention, accordingly, is directed to the, provision of a novel X-raytube producing a beam which scans the object under examination and, inthe new tube, the scanning effect is obtained by causing the electronbeam impinging upon a target surface to scan that surface. The tube isprovided with a cathode having a central opening coaxial with the targetand a filament of circular form and substantial length, which encirclesand is concentricwith the opening. The tube includes an X-raytransparent window coaxial with the target and the cathode opening, andfocusing means, which cause the electrons emitted by the cathode toconverge along curved trajectories and combine into a beam directedtoward the target. The beamis acted on by deflecting means, which causethe beam to scan the target surface. X- rays emitted by the target thenpass through the opening in the cathode and through the window, and aray-proof element having a small opening in the common axis of thecathode and the target and serving as an image forming means is disposedin the path of the X-rays. Accordingly, as the electron beam scans thetarget, the X-ray beam passing through the small opening in theray-proof element scans the object in the reverse sense. All parts ofthe object within the field of the X-ray ,beam are thus traversedconsecutively by the X-rays.

A form of the new discharge device useful in projection televisionovercomes certain objec- 1 tions to present systems, as follows. Themost satisfactory projection television systems now in use employ aSchmidt optical system, which comprises a spherical concave mirrorexternal to and aligned with; the fluorescent screen of the tube andanaspheric corrector plate at the center of curvature of the mirror. Inaddition to being expensive because of the added cost of the opticalelements, such a system occupies much space, since the mirror is usuallyat least twice the di-: ameter of the screen and the aspheric plate en:

circles the neck of the tube and has approximately the same diameter asthe mirror. Thus, the complete unit has overall dimensions much greaterthan and varying with the size of the tube used.

In the application of the discharge device of the invention fortelevision projection purposes, the target surface of the device isprovided with a. suitable phosphor coating and the envelope of thedevice is provided with a window transparent to visible light andaligned axially with the target. Beyond the window and coaxial therewithis mounted a lens system for concentrating and focusing the light fromthe target to form an image upon a viewing screen remote from thedevice. Since the useful light does not pass through the target surfaceof the new device, the target can be made of metal and provided, ifnecessary, with cooling means. The electron beam used may, accordingly,be of higher intensity without doing damage to the target surface andthe increased light thus obtained compensates for inefficiency of theoptical features.

For a better understanding of the invention, reference may be made tothe accompanying drawing, in which Fig. 1 is a view in longitudinalsection through one form of X-ray tube embodying the invention; and

Fig. 2 is a plan view of part of the tube of Fig. 1;

Fig. 3 is a longitudinal section of a tube embodying the invention andsuitable for use in projection television;

Fig. 4is a fragmentary longitudinal sectional view through a tube of theinvention provided with alternative modulating means; and

Fig. 5 is a plan view of part of the tube shown in Fig. 4.

The tube illustrated in Fig. 1 comprises an evacuated envelope in havingan enlarged section ll formed in part by a glass wall l2, on one openend of which is sealed a tubular metal element l3 terminating at itsouter end in a radial flange l3a. A target anode assembly extends intothe enlarged section II of the envelope through the open end of elementl3, and the assembly includes a heavy block of metal l4, shown as havinga concave target surface coaxial with the envelope and facing inward.The targetsurface may be formed of a sheet 15 of'tungsten embedded inthe surface of block II, which may be of copper, or the target surfacemay be formed by a thin gold plating on the face of the block. The blockl4 may be provided with embedded cooling coils I6, through which acoolant supplied through tubes l1, l8 may be circulated. The block I4 ismounted in a tubular metal member is having a radial flange Isa at itsfree end, the flange lying in contact with flange l3a and being securedthereto as by brazing, indicated at 20. If desired, a flat ring may besecured to the upper surface of flange IS, the ring having an innerdiameter somewhat less than that of the ,tubular member l9, or the ringmay be replaced by a disc 2| having openings, through which the tubesl1, l3 extend. At its inner end, the block l4 carries a projectinghollow shield 22 having a rolled edge 220.

A tubular metal member 23 is sealed in the end of the glass section l2opposite to that in which the anode structure is mounted, and a rayproofplate 24 having a central opening is ,secured to the end of member 23. Ahollow neck 25 of tapering diameter and made of ceramic or otherdielectric material is secured in place by of the flange.

metaiized ceramic hard soldering processes with its large end fixed inthe opening in plate 24. Within the enlarged section H of the tube, theneck has a metallic extension 25a with an outwardly rolled edge, theextension having a radial flange at its small end secured to plate 24.The small end of neck 25 is seated upon and secured to the end wall 26of the end section 21 of the envelope by means of a. flanged collar 26a.

The wall 26 is formed with a central opening defined by an internalflange 28, which is coaxial with the neck and the target and contains aberyllium diaphragm 28a having a central opening. Flange 28, neck 25,and extension 2511 are coated with a continuous thin high resistanceconductive coating such as that known commercially as Aquadag. Endsection 21 includes a hollow glass section 29 sealed at one end to acylindrical flange at the outer periphery of wall 26 and, at its otherend, to one end of a hollow metal member 30 having a. radial flange 3|at its other end. A hollow metal member 32 lies beyond member 30 and atone end has a radial flange 33 lying in contact with flange 3| andsecured thereto by brazing. The other end of member 32 is seated againstand secured to a plate 34 having a tubular extension 35 of substantiallyless diameter than members 30 and 32 and extending through those membersto terminate close to end wall 26. The extension has a central openingprovided with an internal flange 35a and a window 36 of X-ray'transparent material, preferably beryllium, is mounted in the openingagainst the inner face A ray-proof lead diaphragm 31 having a centralpinhole 31a is mounted in the opening against the outer face of flange35a and a filter 38 lies against the outer face of the diaphragm 31. Thefilter and diaphragm are held in place by an externally threaded annularnut 39 screwed into the opening.

The portion of the extension 35 inwardly from window 36 forms a cathodegenerally designated 40 and, at its inner end, the cathode is providedwith a flange 4| of L-section, which, with the wall of the extension,forms a circumferential channel/encircling flange 28 and spacedlaterally therefrom. A filament 42 of generally circular form is mountedin the channel on supports 43' and a lead 44 is connected to one end ofthe filament and extends outwardly through an insulating bushing 45through the cathode wall. The lead is connected to a conductor 46, whichlies between extension 35 and members 3| and 32 and is connected to alead 41 sealed through-an insulating bushing 48 mounted-in an opening inthe wall of member 32. Lead 41 is connected to a source 49 of filamentcurrent.

A focusing coil 50 in'casing 5| of paramagnetic material encircles thehollow neck 25 of the envelope near its small end. The coil is suppliedwith energy from a source 52. Deflecting coils 53 enclosed in insulationand of the type used in television receiver tubes, encircle the neck 25between the focusing coil and plate 24. The deflecting coils areenergized by means of conventional sweep circuits (not shown).

The means for supplying energy to the tube are diagrammaticallyillustrated as including sources 54 and 55 connected in series. One sideof source 54 is grounded and the other side is connected to source 55and to the end wall 26 of the end section 21 of the envelope end to thehollow neck 25. Theother side of the larger source 55 is connected tothe anode assembly. Plate I4 is rounded.

aseaava In the operation of the X-ray tube, electrons emitted byfilament I! are focused by the walls of the channel, in which thefilament lies, and their initial travel out of channel is in a directionaway from the anode. The inner surface of the wall 58 of the cathodeandwindow I. deflne a concavity, the surface of which is at the samepotential as the filament. and the wall and window act as focusingmeans, which cause the electrons to :travel along convergent curvedtrajectories. so that the electrons pass through the opening indiaphragm Ila and through hoilow flange 20. The flange, the ,diaphragm,and focusing coil 50 act to form theelectrons into a beam, which travelsalong the common axis of neck and the target toward the latter. Neck 25cannot be made ofunetal because eddy currents induced by the scanningcoil would cause overheating. Because it is desirable to maintainuniform potential along this portion, however, a thin high resistanceconductive coating is applied to the inner surface of the neck. Bymaking the coating thin and of high resistance material. eddy currentsin the coating are negligible. The energization of the deflecting coils53 by the sweep circuits causes the electron beam to scan the targetsurface of the anode and. because of the ray-proof disc 31 and plate 24,the only X- rays which can issue from the tube in the general directionof the axis are those passing through the pin hole 31a in the disc.Accordingly, as the electron beam scans the target surface. the X- raybeam issuing from the pin hole has a scanning movement opposite to thatof the electron beam. The parts of the object to be examined lyingwithin the fleld of the X-ray beam are, accordingly, consecutivelyirradiated in accordance with the scanning sequence determined inconventional manner by the design of the sweep circuits.

The modified form of the electron device of the invention shown in Fig.3 is similar to that shown in Fig. 1, except for the target surface, thewindow, and the provision of optical elements outside the window and ofmeans for modulating the intensity of the electron beam.

The Fig. 3 tube comprises an evacuated envelope having an enlargedsection I i' of the same construction as section II of the Fig. 1 tube.A target anode assembly extends into section it and the assemblyincludes a block H of metal having a concave target surface coaxial withthe envelope and facing inward. The target surface has a phosphorcoating forming a screen II, which fluoresces when electrons impingethereon, and the block may be provided with embedded cooling coils l6for circulation of a coolant supplied through tubes ll, II. The blockcarries a hollow shield 22' similar to shield 32.

The inner end of the section II of the tube is attached to an opaqueplate 24' having a central opening and a hollow neck 25 of ceramic orother dielectric material and of tapering diameter extends through theopening with its large end mounted in place in the opening. The neck hasa metallic extension 25a mounted by means of a flange on plate 24 withinsection Ii and the small end of the neck is seated upon and secured tothe end wall 26 of the end section 21' of the envelope by means of aflanged collar 28a.

The wall 26' is formed with a central opening deflned by an internalflange 28' coaxial with the neck and the target, and flange II, theinner surface of neck 2!. and the neck extension Ila are coated with acontinuous thin high resistance through the cathode wall.

conductive coating of a material, such as that known commercially asAquadag." The end section 21' of the envelope is similarin constructionto end section 21 of the tube of Fig. 1 and has a tubular extension I!similar to extension I! but internally threaded throughout substantiallyits entire length. At its inner end, extension 35' has a central openingdefined by an internaL flange 35a, and a glass window ll is mounted toclose the opening and is sealed vacuum-tight to a washer II of the metalknown commercially as Kovar" or some other suitable metal. the washerbeing in turn brazed to the flange inwardly from the window.

A pairs of lens mounts are threaded into extension 35' and the innermount includes a ring I! having a central opening defined by anexternally threaded flange. A lens element 59 is mounted to close theopening through ring it and is held in place against the flange by a capoverlying the element and threaded on the outside of the flange. Thesecond lens mount comprises an externally threaded tubular member 8|carrying a lens element 62 at its outer end, which is held in place by acap 83 overlying the element and threaded on the end of member if.

The portion of extension 35 inwardly from window 3i forms a cathodegenerally designated 40' and, at its inner end, the cathode is providedwith a flange ll of L-section which, with the wall of the extension,forms a circumferential channel encircling flange 28' and spacedlaterally therefrom. A filament 42' of generally cir cular form ismounted within the channel on leads 43', N, which extend outwardlythrough insulating bushings 45 mounted in openings The leads areconnected to conductors 46', which lie within the end section 21 betweenthe extension 35 and the outer wall of the section. Conductors 46" areconnected to leads 1' sealed through insulating bushings 48' in openingsin the wall of section 21'. The leads 4'! arejconnected, respectively,to the positive and negative terminals of a source of fllament current.

A focusing coil 50' in a casing of paramagnetic material encircles theneck 15' near its small end and is supplied with energy from a source52'. Deflecting coils 53' enclosed in insulation and of the type used intelevision receiver tubes encircle neck 25' between the focusing coiland plate ll. The deflecting coils are energized by means ofconventional sweep circuits (not shown) The tube of Fig. 3 is suppliedwith energy in the same manner as the tube of Fig. 1, the end plate 34',which is integral with extension I". being grounded and potential beingmaintained on plate 26' and the anode assembly. The tube is providedwith means acting on the electron beam to modulate the intensity thereofand, in the construction illustrated, the modulating means comprises atransformer 64, the secondary of which is connected between the negativefllaxnent lead 41 and a suitable part of the assembly, such as the plate34', which has a good electrical connection with the cathode channel llThe operation of the tube of Fig. 3 is generally similar to that of thetube of Fig. 1, except that. when the electron beam strikes thefluorescent screen i5, visible light instead of X-rays are produced. Thelight passes from the screen through neck I! and window 36' and is thencondensed by the lens system consisting of elements 5! and I. The imageproduced upon the screen is pro- Jected upon an external viewing screen.

asses-m "-Iii the construction illustrated, the concave surface of thecathode '40 in front of the filament channelacts as a focusing means forthe electrons emitted by the filament. and, when no signal is impressedon the primary of the transformer 64, the cathode and such focusingmeans are at the same potential as the filament. When a'signal isimpressed on the transformer priem'ary,-the potential of the cathodefocusing means is changed relative to that of the filament, with theresult that moreor less electrons are de-' flected to pass through theopening in flange 28'. the intensity of the light produced at the screenis proportional to the density of the electron stream impinging on thescreen, the amount of light produced at any point on the screen maybejcontrolled' by impression of suitable signalson the primary f themodulation means. i

' It" is to be understood that the modulation means "described above ismerely by way of example, and any other means, which producesa'variation in the potential gradient at the electron-emitting surfaceof the cathode in accordance"withthe modulating signal, may be employed'. Another example of such modulating means is illustrated inFigs. 4 and 5-and it involves the use of agrid 65 made of a pair ofcircular concentric wires 86, 61 connected by radial tie wires 8'. Theouter circular wire 61 is provided with radial pins 69 and the grid ismounted in the cathode do" with pins 61 embedded in msulating supports10 inserted in bores in the wall of the cathode. One of the pins 69 isconnected by a conductor II to a lead 12 connected to one side of thesecondary of a transformer 13,-the

other side of the secondary being connected to' the grounded plate 34".The grid is so positioned that the electrons emitted by the filament 42"pass through the grid between the wires thereof and, in accordance withthe modulation signal impressed thereon, the grid permits a varyingnumber of electrons to pass through it, sothat the intensity of theelectron beam is correspondingly varied. 1 I In both forms of the newdischarge device, the flange 2!, 28' serves as the first anode andperforms the dual functions of focusing and accelerating the electrons.The continuous conductive coating on the inner surfaces of the flange212i, the neck 25, and the neck extension fie, 25a keeps the entireregion between enlarged section II, If and end section 21, 21' atconstant potential. so that the electron beam is not distorted." Thetarget is maintained at a higher positive potential than flange 28, 28'in order to cause further acceleration of the electrons and producegreater effects. For some purposes, however, the electrons may besufliciently acceleratedby the first anode alone and, in that case, thetarget can be maintained at the'same potential as the first anode.

The electron beam may be focused either by an electric field or by amagnetic field, and the,

We claim:'

1. An electron tube, which comprises an evacu ated envelope having awindow for escape of radiant energy, an anode having a target surfacewithin the envelope coaxial with andspaced from the window, cathodemeans within the en-- velope adjacent the window, the cathode meanshaving a central opening coaxial with the target surfaceand window and afilament encircling and concentric with the opening, means for focusin'g electrons emitted by the filament to cause them'to' pass throughsaid central opening and form a beam traveling toward the targetsurface, means outside the tube and acting on the beam between thecathode and the target surface-to cause the beam to scan the targetsurface. and image forming means in the path of the radiant energytraveling from the target surface through the cathode opening forconverting said energy into a usefui beam traveling away from the -en--velope and having a lateral movementsimilar to the scanning movement ofthe electron beam but opposite in sense. v

2. Anelectron tube, which comprises an" evacuated envelope having awindow for escape of radiant energy, an anode having a target surfacewithin the envelope coaxial with and spaced from the window, cathodemeans within the envelope adjacent the window, the cathode means havinga central opening coaxial with the target surface and window and afilament encircling and con-' centric with the opening, said cathodemeans also including. means for focusing" electrons emitted by thefilament to cause'them'to pass through said central opening and form abeam traveling toward the target. surface, means for deflecting the beamto causeitto scan the target surface,. and image forming means .in thepath of radiant energy traveling from the target sur-, face through thecathode opening for converting said energy into a useful beam travelingaway from the envelope and having a lateral movement similar tothescanning movementof the electron beam but opposite in sense.

3. An electron tube, which comprises an evacuated envelope having awindowfor escape of radiant energy, an anode having a target surfacewithin the envelope coaxial with and spaced from the window, cathodemeans within the. envelope adjacent the window, the cathode means havinga central openin'g coaxial with the target surface and window and afilament encircling and concentric with the opening, means both insideand outside the tube for focusing electrons emitted by the filament tocause them to pass through said central opening and form a beamtravelingito ward the target surface, means for deflecting the beam tocause it to scan the target surface, and image formingzmeans in thepathof radiant energy'traveling from the target surface'through the cathodeopening" for converting said energy into a useful beam traveling awayfrom the envelope and having a lateral'movement similar the scanningmovement of the electron beam but opposite 'in sense. V

4 An electron tube, which comprises an evacuated envelope having awindow for escape of radiant energy. an anode having a target surfacewithin the envelope coaxial with and spaced from the window. cathodemeans within the envelope, adjacent the. window. the cathode means havina. central opening coaxial, with thetarget surface and.window and.amamentencimlmg andv concentric with the opening, means for focusingelecing said energy into a useful beam traveling away from the envelopeand having a lateral movement similar to the scanning movement of theelectron beam but opposite in sense.

5. An electron tube, which comprises an evacuated envelope having awindow for escape, of radiant energy, a target anode having a targetsurface within the envelope coaxial with and spaced from the window, thecathode means having a central opening coaxial with the target surfaceand window and a filament encircling and concentric with the opening,means, including a hollow anode, for focusing electrons emitted by thefilament to cause them to pass through said central opening and form abeam traveling toward the target surface, and means outside the envelopeand acting on the beam between the hollow anode and the target surfaceto cause the beam to scan the target surface.

6. An electron tube, which comprises an evacuated envelope having an endsection provided with a wall formed at least in part by a windowtransparent to radiant energy, an enlarged section, and a neckconnecting the end and enlarged sections and diminishing in diameter,toward the end section, an anode in the enlarged section having a targetsurface producing radiant energy when struck by an electron beam, thetarget surface being coaxial with the neck and window, a cathode in theend section having an opening coaxial with the neck and a filamentencircling and concentric with the opening, means for focusing electronsemitted by the filament to cause them to pass through said centralopening and form a beam traveling toward and impinging upon the targetsurface, and means for deflecting the beam to cause it to scan thetarget surface.

'7. An electron tube, which comprises an evacuated envelope having anend section provided with a wall formed at least in part by a windowtransparent to radiant energy, an enlarged section, and a neckconnecting the end and enlarged sections and diminishing in diametertoward the end section, an anode in the enlarged section having a targetsurface producing radiant energy when struck by an electron beam, thetarget surface being coaxial with the neck and window, a cathode in theend section having an opening coaxial with the neck and a filamentencircling and concentric with the opening, means for focusing electronsemitted by the filament to cause them to pass through said centralopening and form a beam traveling toward and impinging upon the targetsurface, and means encircling the neck and operable to deflect the beamto cause it to scan the target surface.

8. An electron tube, which comprises an evacuated envelope having awindow for escape of radiant energy, an anode having a target surfacewithin the envelope coaxial with and spaced from the window, cathodemeans within the envelope adjacent the window, the cathode means havinga central opening coaxial with the target surface i0 and window and afilament encircling and concentric with the opening, means for focusingelectrons emitted by the filament to cause them to pass through saidcentral opening and form a beam traveling toward the target surface,means for deflecting the beam to cause it to scan the target surface,and image forming means in the path of radiant energy traveling from thetarget surface through the cathode opening for converting said energyinto auseful beam traveling away from the envelope and having a lateralmovement similar to the scanning movement of the electron beam butopposite in sense.

9. An X-ray tube, which comprises an envelope having awindow transparentto X-rays, an anode having a target surface within the envelope coaxialwith and spaced from the window, cathode means within the envelopeadjacent the window, the cathode means having a central opening coaxialwith the target surface and window and a filament concentric with theopening, means for and form a beam traveling toward the target surface,means for deflecting the beam to cause it to scan the target surface,and a ray-proof shield in the path of the X-rays leaving the targetsurface and having a small opening on the common axis of the targetsurface and the cathode opening.

10. An X-ray tube, which comprises an envelope having a windowtransparent to X-rays, an anode having a target surface within theenvelope coaxial with and spaced from the window, cathode means withinthe envelope adjacent the window. the cathode means having a centralopening coaxial with the target surface and window and a filamentconcentric with the opening means for focusing electrons emitted by thefilament to cause them to converge along curved trajectories and passthrough said central opening and form a beam impinging upon the targetsurface, means for deflecting the beam to cause it to scan the targetsurface, and a rayproof shield outside the window and having a smallopening on the common axis of the target surface and the cathodeopening.

11. An X-ray tube, which comprises an envelope having a windowtransparent to X-rays, an anode having a target surface within theenvelope coaxial with and spaced from the window, cathode means withinthe envelope adjacent the window, 'the cathode means having a centralopening coaxial with the target surface and window and a filamentconcentric with the opening, means, including the window, for focusingelectrons emitted by the filament to cause them to converge along curvedtrajectories and pass through said central opening and form a beamimpinging upon the target surface, means for deflecting the beam tocause it to scan the target surface, and a ray-proof shield outside thewindow and having a small opening on the common axis of the targetsurface and the cathode opening.

12. An X-ray tube, which comprises an evacuated envelope having an endsection provided with a wall formed at least in part by a metallicwindow transparent to X-rays, an enlarged section, and a neck connectingthe end and enlarged sections and diminishing in diameter toward the endsection, an anode in the enlarged section having a target surfaceproducing X-rays when struck by an electron beam, the target surfacebeing coaxialwith the neck and window, a cathode in the end sectionhaving an opening coaxial with the neck and a filament encircling andconcentric with the opening, means for maintaining potential upon saidwall of the end section to "cause electrons emitted by the filament toconverge and pass through the cathode opening and opening, said elementhaving a small opening on the common axis of the target surface andneck.

13'. A cathode ray tube, ,which comprises an evacuated envelope having atransparent window, an anode having a fluorescen target surface withinthe envelope coaxial with a d spaced from the window, cathode meanswithin'the envelope adjacent the window, the cathode means having acentral opening coaxial with the target surface and window and afilament encircling and concentric with the opening, means for focusingelectrons emitted by the filament to cause them to ,pass through saidcentral opening and form a beam impinging upon the target surface, thelight produced by the target surface traveling through said centralopening and window, means for deflecting the electron beam to cause itto scan the target surface, means for varying the intensity of theelectron beam, and image forming optical means in the path of the lightbeam issuing through the window.

14. A cathode ray tube, which comprises an evacuated envelope having atransparent window, an anode having a fluorescent target surface withinthe envelope coaxial with and spaced from the window, cathode meanswithin the envelope adjacent the window, the cathode means having acentral opening coaxial with the target surface and window and afilament encircling and concentric with the opening, means both insideand outside the envelope for focusing electrons emitted by the filamentto cause them to pass through said central opening and form a beamimpinging upon the target surface, the light produced by the targetvsurface traveling through said central opening and window, means forvarying the intensity of the electron beam, means for deflecting theelectron beam to cause it to scan the target surface, and image formingopticentric with the opening, means both inside and outside the envelopefor focusing electrons emitted by the filament, the internal focusingmeans causing the electrons to converge along curved paths and passthrough the cathode opening and the external focusing means acting onthe electrons thereafter to form them into a beam impinging upon thetarget surface, the light produced by the target surface travelingthrough the cathode opening and window, means for varying the intensityof the electron beam, means for deflecting the electron beam to cause itto scan the target surface, and image forming optical means in the pathof the light beam issuing through the window.

16. A cathode ray tube, which comprises an v evacuated envelope havingan end section with a metallic wall having an opening closed by atransparent window, an enlarged section, and a neck connecting the endand enlarged sections and diminishing in diameter toward the endsection, an anode having a fluorescent target surface within theenlarged section and facing the large end of the neck, the targetsurface being coaxial with theneck, a cathode in the end section havingan opening coaxial with the neck and a filament surrounding andconcentric with the opening, the cathode also including focusing meansfor directing electrons emitted by the filament away from the anode.means for maintaining potential upon said wall of the end section tocause the electrons to converge and pass through the cathode opening andenter the neck, focusing means acting on the electrons within the neckand forming them into a beam impinging upon the target surface,deflecting means acting on the electron beam and causing it to scan thetarget surface, the light produced by the target surface travelingthrough the neck, cathode opening, and window, and image forming opticalmeans in the path of the light beam issuing through the window.

17. A cathode-ray tube, which comprises an evacuated envelope having anend section with a metallic wall having an opening closed by atransparent window, an enlarged section, and a neck connecting the endand enlarged sections and diminishing in diameter toward the endsection, an anode having a fluorescent target surface within theenlarged section and facing the large .end of the neck, the targetsurface being coaxial with the neck, a cathode in the end section havingan opening coaxial with the neck and a filament surrounding andconcentric with the opening, the cathode also including focusing meansfor directing electrons emitted by the filament away from theanode,.means for maintaining potential upon said wall of the end sectionto cause the electrons to converge and pass through the cathode openingand enter the neck, focusing means acting on the electrons within theneck and forming them into a beam impinging upon the target surface,means for varying the potential maintained upon said wall to vary thenumber of electrons entering the neck and thereby vary the intensity ofthe electron beam, deflecting means acting on the electron beam andcausing it to scan the target surface, the light produced by the targetsurface traveling through the neck, cathode opening, and window, andimage forming optical means in the path of the light beam issuingthrough the window.

18. An electron tube, which comprises an evacuated envelope having awindow for escape of radiant energy, an anode having a targetsurfacewithin the envelope coaxial with and spaced from the window,cathode means within the envelope adjacent the window, the cathode meanshaving a central opening coaxial with the target surface and window anda filament encircling and concentric with the opening, the cathode meansincluding walls acting to cause electrons emitted by the filament toconverge along curved trajectorles and pass through said centralopening, a hollow anode acting on the electrons as they approach saidcentral opening to form them into a beam traveling toward the targetsurface, means acting on the beam between the hollow anode and thetarget surface and causing the beam to scan the target surface, andimage forming means in the path of radiant energy traveling from thetarget surface through the cathode opening for converting said energyinto a useful beam traveling away from the envelope and havina a lateralmovement similar to the scanning movement of the electron beam butopposite in sense.

19. In an electron tube having an evacuated envelope, the combination ofan anode having a target surface within the envelope, cathode meanswithin the envelope having a central opening coaxial with the targetsurface and a filament encircling and concentric with the opening, thecathode means including means for causing electrons emitted by thefilament to con- JOSEPH w. SKEHAN. moms H. ROGERS.

REFERENCES CITED I The following references .are'of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 1,685,928 Morrison Oct. 2, 19281,946,288 Kearsley Feb. 6, 1934 2,467,462 Brown. Jr. Apr. 19, 1949

